Variable reactances



May 24, 1960 F. H. GALE VARIABLE REACTANCES 2 Sheets-Sheet 1 Filed Dec. 9, 1955 FIG. 3.

l raven-boz FHGae May 24, 1960 F. H. GALE 2,938,181

VARIABLE REACTANCES Filed Dec. 9, 1955 2 Sheets-Sheet 2 2,938,181 .Pa tented May 24, 1960 2,938,181 VARIABLE REACTANCES Frederick Henry Gale, Hillingdou, England, assignor to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Filed Dec. 9, 1955, Ser. No. 552,208

Claims priority, application Great Britain Dec. 18, 1954 4 Claims. (Cl. 333-83) This invention relates to variable reactances and in particular to variable react-ances used in waveguides or resonant cavities.

The object of the present invention is to provide a variable reactance of simple and inexpensive construction.

According to the present invention there is provided a variable reactance in the form of a vane twisted about an axis and mounted in a waveguide of rectangular crosssection so that the vane can be rotated about said axis relative to said guide to cause the section of the vane most liable to reflect electro-magnetic waves in the waveguide to be displaced along said axis. The section of the vane most liable to reflect electro-magnetic waves is that parallel to the electric field in the guide and as the vane is rotated this section moves along the waveguide by virtue of the twist of the vane. An efiect is produced, therefore, which is similar to that obtained by varying the length of a short circuited waveguide.

In one form of the invention the vane is mounted in the guide so as to form therewith a resonant cavity and means are provided for adjusting the angular setting of the vane to vary the resonant frequency of said cavity.

In another form of the invention the vane is mounted for continuous rotation about said axis so that when electromagnetic Waves are incident on the vane, waves are reflected from the vane differing from the incident waves in frequency in dependence upon the rate of rotation of the vane, thereby to simulate the doppler effect which arises when waves are reflected from a moving body. This form of the invention may therefore be called a doppler simulator, and is useful for testing purposes.

The vane may be in the form of a helically twisted strip but other structures which are electrically equivalent may also be used, such as a series of rods or strips angularly disposed about the axis of twist and spaced in succession along said axis. In general therefore the vane may comprise any structure, which, at successive points spaced along said axis, presents elongated sections transverse to and angularly displaced one with respect to the other about said axis, so that when the structure is rotated the effect is produced of displacing the section which is parallel to a predetermined direction along said axis.

In order that the invention may be clearly understood and readily carried into efiect, the invention will be described with reference to the accompanying drawings.

Figure 1 shows one application of the invention in which the vane is mounted adjacent to a short circuited end of a waveguide,

Figure 2 shows a reactance according to the present invention used as a doppler simulator, and

Figure 3 shows a modification of Figure 1.

Referring to Figure l, the reference 1 shows a fragment of a waveguide of rectangular cross-section and short circuited at its end by an end plate 2, so that it can be used as a resonant cavity. A vane 3 is mounted in the waveguide adjacent the short circuited end and the vane has an electrical length of one half wavelength at the centre frequency of the waves to be set up in the waveguide. The

vane is supported by a shaft 4 which is rotatably mounted in a bearing on the end plate 2 and projects to the exterior of the waveguide where it carries an adjusting knob 5 and a pointer 6. The vane 3 has a degree twist about its longitudinal axis 7 and this axis is parallel to the direction of the principal electric vector in the waveguide. The width of the vane to give the best reflection is found empirically for the particular Waveguide used. With 90 degrees twist in a vane having an electrical length of one half wavelength, rotation of the vane between 0 degrees and 90 degrees from a given datum position gives positive and negative reactances. The pointer 6 co-operates with calibrations on the end plate 2 of the waveguide, and the calibration may indicate the value of the reactance at a predetermined frequency or the resonant frequency of the cavity as desired.

The invention is not confined to vanes having a 90 degree twist in an electn'cal'length of one half wavelength and in the doppler simulator shown in Figure 2, the vane 3 is twisted through degrees in an electrical length of one half wavelength. In the case of Figure 2, the vane is mounted on an insulating frame 8 which is slidable in the waveguide 1 as indicated. The vane is mounted so that it can rotate freely about its longitudinal axis, which is parallel to the direction of propagation of the waveguide. The vane 3 is shaped so that the phase of the reflecting section varies approximately linearly when the vane is rotated and in this form of the invention rotation of the vane is produced by supplying compressed air along the waveguide as indicated by the arrow 9. The incident electromagnetic energy is directed along the guide in the direction of the arrow '10. The frame 8 is attached by cord 11 to a tail 12 of magnetisable material and a permanent magnet 13 is fitted on the waveguide for holding the frame 8 in a desired position from the exterior of the waveguide, by exerting a magnetic force on the tail. In the absence of the magnet, the air pressure tends to blow the frame 8 along the guide 1. A second magnet 14 may also be used as shown when it is desired to move the frame, and in this case on removing the first magnet, the frame is blown along the guide until it is stopped by the second magnet.

With an arrangement such as illustrated in Figure 2, a simulated doppler signal approximating to a sine wave can be produced and doppler frequencies (that is the frequency difference between the incident and reflected waves) greater than 500 cycles per second can be readily achieved.

If a reactance of larger dimensions than indicated in Figures 1 and 2 is desired, it may be more convenient to construct it, as shown in Figure 3, of a series of rods or strips 15 mounted on the shaft 4 so that they project radially to both sides of the shaft, the rods being angularly disposed about the axis of the shaft 4 and spaced in succession along the axis of the shaft 4. The resultant structure is then equivalent to a vane formed by twisting a single piece of metal as in Figures 1 and 2, and other electrically equivalent structures may also be employed.

Where the invention takes the form of a tunable cavity, the vane will in general be formed of metal but where the invention is in the form of a doppler simulator vanes of insulating material may sometimes be used, for example if low reflection coefiicients are desired for test purposes.

What I claim is:

l. A variable reactance in the form of a vane twisted about an axis and mounted in a waveguide of rectangular cross-section so that the vane can be rotated about said axis relative to said waveguide to cause the section of the vane most liable to reflect electro-magnetic waves in the waveguide to be displaced along said axis.

2. A variable reactance according to claim 1 wherein said vane is mounted in the guide so as to form therewith a resonant cavity, and mean's' 'are proi/ided for adjnting the angular setting of the vane to vary the resonant frequency of said cavity,

3. A variable reactance according to claim 1 wherein the vane is mounted'f'or contim'ious rotation'about said 'axis and means are provided for rotating said vane to produce reflected waves from the vane difiering in frequency from the corresponding incident. waves in dependence upon the rate of rotation of the vane.

4. A van'able reactance according to claim 3, wherein said means for rotating said vane comprises means for blowing air through the waveguide.

' References Cited in the file of this patent UNITED STATES PATENT S Zaieski Feb. 10, 1953 Clogston Mar. 3, 1953 Kock Sept. 7, 1954 Meisenheimer et a1 Apr. 19, 1955 Jacobsen et a1 s Aug. 7, 1956 Whitehorn Feb. 26, 1957 FOREIGN PATENTS Great Britain Jan. 16, 1952 

